Printable film

ABSTRACT

The invention relates to printable films comprising a substrate and at least a surface layer, said layer covering at least one face of said substrate and comprising a water-dispersible polymer and an ethylenically unsaturated compound; to a process for the manufacture of such films; to printed films and especially to printed labels obtained from such printable films.

FIELD

This application is a continuation application of U.S. patentapplication Ser. No. 09/117,214, filed Jul. 24, 1998, now abandoned,which is a 371 of PCT/BE97/008, filed Jan. 21, 1997. The entirety of allof the aforementioned applications is incorporated herein by reference.

BACKGROUND

The present invention relates to an improved printable film having goodink adhesive properties and relates more particularly to an improvedprintable film having good adhesive properties when used with radiationcurable ink.

In recent years, diversification of printed products has requiredprinting on a wider variety of materials in sheets; for example, papers,synthetic papers, polymer films such as thermoplastic resin films,metallic foils, metallized sheets, etc. These printed items are printedby methods such as by offset printing, gravure, flexography, screenprocess printing and letterpress printing. In these printing methods, amethod which uses radiation curable ink has recently become popularbecause radiation curable inks cure rapidly, and the printing methodwhich uses radiation curable ink is of superior handling. Radiationcurable inks are known to be useful in the printing of packaging, labelsand non absorbing printing materials. Radiation curable printing inkstypically contain unsaturated acrylates, polyesters, photoinitiators,and additives. In electron beam cured inks however, the photoinitiatorsmay be omitted.

After deposition of the radiation curable ink on the printable item, theprint is exposed to radiation and hardens within a fraction of a second.Printing speeds up to 300 m/min are attained during continuous printing.At present, there is a great demand for sheet-like printable items.

In printing methods, the printing sheet requires sheet runningproperties anti-blocking properties, producing uniform spread of the inkover the surface of the sheet, as well as antistatic properties. Besidesthese generally required properties, in printing methods which useradiation curable ink, the printing sheet requires in particular theproperty of adhering strongly to radiation cured ink.

In particular, radiation curable ink printed polymer films, intended foruse as labels, for example in the bottle labelling market, should beresistant to both freezing water conditions (i.e. storage in coolers orice buckets for 24 hours) and boiling water conditions (i.e.pasteurization by immersion in water at 95° C. for up to 1 hour).

European patent application EP-A1-410051 discloses printing sheetscomprising a support layer and a surface layer on at least one face ofsaid support, said surface layer containing at least an acrylate basedpolymer and an unsaturated compound (cinnamic acid or derivativesthereof).

Neither this document, nor any other documents of the state of the artdo teach anything about the possibility of use of other monomers toreplace cinnamic acid.

However, the above listed materials formed in sheets, especially polymerfilms do not sufficiently adhere to radiation curable ink after printingand curing, especially in these extreme conditions. Accordingly theprinted and radiation cured ink has a problem in the fact that theprinted and radiation cured ink separates from the polymer film.

Therefore, a printable film which has superior adhesion to the radiationcurable ink, even in extreme conditions, is required.

DETAILED DESCRIPTION

The present invention has the above-described problems in mind, and anobject of this invention is therefore to provide a printable film,particularly a printable polymer film, which has the superior propertiesrequired of printable films, and particularly has superior adhesiveproperties with radiation cured ink.

Accordingly, the present invention, provides a printable film comprisinga substrate and at least a surface layer which covers at least one faceof the substrate and which comprises a water dispersible polymer and anethylenically unsaturated compound.

Suitable substrates, which can be used in this invention, are polymerfilms, especially polyolefin films, papers, synthetic papers, wovenfabrics, nonwoven fabrics, ceramic sheets, metallic plates, andmultilayer composite sheets formed by combination of said materials. Forprintable film intended for use as labels, polyolefin films arepreferred, especially oriented polypropylene films, and still morepreferred is an oriented polypropylene film according to the EuropeanPatent Application 202812.

In the present specification, the expression “printable film” denotes afilm which is capable to be directly inked, i.e. a film of which thesurface layer is strong enough to resist the pull of the tacky ink,otherwise areas of the surface layer may be pulled away from thesurface, giving a defect known as picking.

The expression “at least a surface layer” denotes a layer which coversat least one face of the substrate listed above. This surface layercomprises a water dispersible polymer, for example a water dispersibleacrylic or urethane polymer. In the present specification, an “acrylicpolymer” means a (co)polymer obtained by the free-radical additionpolymerization of at least one (meth)acrylic type monomer and optionallyof other vinylic or allylic compounds. It is essential that this acrylicpolymer should be able to provide a smooth film-formed and reasonablyopen surface.

A wide variety of acrylic polymers are able to fulfill this requirement.Suitable acrylic polymers are homopolymers of (meth)acrylic acid oralkyl (meth)acrylate, the alkyl radical having 1 to 10 carbon atom, orcopolymers of two or more of the said (meth)acrylic type monomer andoptionally of other vinylic or allylic compounds.

As said above, a water dispersible urethane polymer may also suitably beused. As with the acrylic polymer, it is essential that this urethanepolymer should be able to provide a smooth film-formed and reasonablyopen surface.

A wide variety of urethane polymers are able to fulfill thisrequirement. Suitable urethane polymers are for example the reactionproduct of an isocyanate-terminated polyurethane prepolymer formed byreacting at least an excess of an organic polyisocyanate, an organiccompound containing at least two isocyanate-reactive groups and anisocyanate-reactive compound containing anionic salt functional groups(or acid groups which may be subsequently converted to such anionic saltgroups) or non-ionic groups and an active hydrogen-containing chainextender.

The surface layer comprises also an ethylenically unsaturated compound.

The ethylenically unsaturated compound is selected to be miscible in thewet stage in the aqueous phase and to be compatible in the dry stagewith the water dispersible polymer itself. Consequently, theethylenically unsaturated compound acts as a plasticiser for the surfacelayer once hardened allowing the easy penetration of the radiationcurable ink thereto.

The ethylenically unsaturated compound must also be able, when theprinted film is submitted to radiations in order to cure the ink, toreact with the unsaturated components of the ink which has penetratedinto the surface layer.

This reaction between the ethylenically unsaturated compounds of thesurface layer and the unsaturated compounds of the radiation curable inkforms chemical bonds between those compounds and simultaneouslycrosslinks the surface layer, thereby generating the final resistantproduct.

Preferably, the ethylenically unsaturated compound contains 1 to 10ethylenical bonds per molecule and still more preferably 2 to 5ethylenical bonds per molecule.

Suitable ethylenically unsaturated compounds are the ester derivativesof α,β-ethylenically unsaturated acids, such as acrylic or methacrylicacids, itaconic or citraconic acids, maleic or fumaric acids, etc. withpolyols or alkoxylated polyols.

The suitable polyols include saturated aliphatic diols such as ethyleneglycol, diethylene glycol, triethylene glycol, propylene glycol,dipropylene glycol, tripropylene glycol, butylene glycols, neopentylglycol, 1,3- and 1,4-butane diols, 1,5-pentane diol, 1,6-hexanediol and2-methyl-1,3 propanediol. Glycerol, 1,1,1-trimethylolpropane, bisphenolA and its hydrogenated derivatives may also be used. The suitablealkoxylated polyols include the ethoxylated or propoxylated derivativesof the polyols listed above.

Examples of ethylenically unsaturated compounds which can be usedaccording to the invention are polyfunctional acrylates such asdifunctional acrylates, such as 1,4-butane diol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, triethylene glycoldiacrylate, polyethylene glycol diacrylate, tripropylene glycoldiacrylate, 2,2-dionol diacrylate, bisphenol A diacrylate, etc.,trifunctional acrylates such as pentaerythritol triacrylate,trimethylolpropane triacrylate, etc. tetrafunctional acrylates, etc.

It is to be understood that the methacrylate derivatives correspondingto these acrylate derivatives could also be used.

Moreover, it has been found that polyallyl derivatives such astetraallyloxyethane are also suitable. Preferably ethoxylatedtrimethylolpropane triacrylate (EBECRYL 1160 from UCB CHEMICALS) isused.

The amount of the ethylenically unsaturated compound can be of fromabout 2 to about 90% by weight of the acrylic polymer, and preferably isfrom about 2 to about 15% (in the present specification, all percentagesare dry weight based).

In order to improve the hardness and/or water resistance of the surfacelayer deposited on the film and consequently, of the finished product, acrosslinking agent may advantageously be added to the surface layer.However, it is to be noted that such a crosslinking agent should bechosen so as the surface layer, once hardened, allows the easypenetration of the radiation curable ink thereto.

Coordinating metal ligands which can form stable coordinated structureswith carboxy or carbonyl functionality perfectly fulfill thisrequirement. Ammonium zirconium carbonate (stabilised or not) isparticularly preferred. The amount of the crosslinking agent can be upto 5% by weight of the acrylic polymer, and preferably is from 1 to 5%by weight of the acrylic polymer.

The surface layer can contain all other additional agents, if necessary,for preventing the blocking of one sheet to another, and for improvingthe sheet running property, antistatic property, nontransparencyproperty, etc. These additional agents are generally added in a totalamount not exceeding about 40% by weight of the acrylic polymer. As saidadditional agent, for example, a pigment such as polyethylene oxide,silica, silica gel, clay, talc, diatomaceous earth, calcium carbonatecalcium sulfate, barium sulfate, aluminium silicate, synthetic zeolite,alumina, zinc oxide, titanium oxide, lithopone, satin white, etc. andcationic, anionic and nonionic antistatic agents, etc. may be used.

According to the present invention, the surface layer may be applied asan aqueous dispersion at about 0.5 to about 2.5 g/m² on the substrate bythe method of roll coating, blade coating, spray coating, air knifecoating, rod bar coating, reverse gravure, etc. on the substrate andthen dried, for example, in a hot air oven.

After the drying step, the surface layer comprises thus the waterdispersible polymer, smoothly crosslinked by the crosslinking agent and,included in the acrylic polymer matrix, the ethylenically unsaturatedcompound.

As said above, this allows the easy penetration of the radiation curableink into the surface layer as well as its subsequent reaction with theethylenically unsaturated compound.

Before applying the surface layer, the surface of the substrate can befirst pretreated in a conventional manner with a view to improve itsadhesiveness. For this purpose, it is possible, for example, to pretreatthe substrate by the corona effect but it should be understood that allknown techniques aiming at improving the surface of a sheet-like itemwith a view of the application of a composition, may be suitable.

It has been observed that in certain cases, the surface layercontemplated herein do not adhere well to film surfaces even when thelater have been subjected to well-known pretreatment operations such as,for example, treatment by corona discharge, flame, or oxidizingchemicals. It has been found, however, that the use of primersintermediate between the substrate and the surface layer provides a highlevel of adherence.

Indeed, in applying coating layer, to a polyolefin film substrate it isgenerally desirable first to apply an intermediate primer or anchorcoating layer to the substrate to ensure adequate adhesion between thesubstrate and subsequently applied coating layer. Although a filmaccording to the invention may be utilized as such a substrate per se(for example, a nonprimed printable film according to the invention isable to resist the freezing water conditions), it is preferred toutilise as a substrate for a subsequently applied layer a film to thereceptive surface(s) of which has been applied a primer or anchorcoating layer.

Advantageously, the film comprises thus further a primer layer betweenthe substrate and the surface layer(s).

As examples of suitable primers, polyethylene imine or polyurethaneacrylate primers crosslinked by isocyanate, epoxy, aziridine or silanederivatives may be cited.

The primer resin may be applied by conventional coating techniques—forexample, by a gravure roll coating method. The resin is convenientlyapplied as a dispersion or as a solution. Economically it would bepreferable to apply the resin as dispersion in water. Aqueous dispersiontechniques have the added advantage that there is no residual odour dueto the solvent present which is generally the case when an organicsolvent is used. However, when using aqueous techniques it is usuallynecessary to heat the film a higher temperature to dry off thedispersant than with systems using an organic solvent or dispersant.Furthermore, the presence of a surfactant, which is generally used toimprove the dispersion of the coating in water, tend to reduced theadhesion between the resin and the base film. Thus, it is also possibleto apply the resin from an organic solvent or dispersant. Examples ofsuitable organic solvent include alcohols, aromatic hydrocarbonsolvents, such as xylene, or mixtures of such solvents as isappropriate.

A reverse face of the substrate, namely a face not covered by thesurface layer, can be covered with a pressure-sensitive adhesive layerwhich consists of a commonly used pressure-sensitive adhesive agent.Furthermore, if necessary, a releasing film or sheet consisting of areleasing agent, can cover the pressure-sensitive adhesive layer. Thislaminate comprising the printing sheet according to the invention can beused as an adhesive label which may be affixed to most types of surface.

Another aspect of the invention concerns thus a printable film intendedfor labels comprising a substrate of which only one face is coated witha surface layer and of which the other face is coated with apressure-sensitive adhesive layer which is itself covered with areleasing film or sheet.

Another aspect of the present invention relates to a process for themanufacture of a printable film comprising the step of coating at leastone side of a substrate with an aqueous dispersion comprising a waterdispersible polymer and an ethylenically unsaturated compound andoptionally a crosslinking agent and/or conventional additives, andcomprising further the step of drying the coating so obtained.

Advantageously, the process of manufacture of a printable film comprisesbefore the step of coating at least one face of the substrate, a furtherpretreatment step of the substrate (such as corona discharge treatment)and still more advantageously a step of priming of the substrate with aprimer.

In a particular embodiment directed to the preparation of labels, onlyone face of said substrate is coated with a surface layer and the oneface-coated substrate so obtained is coated with a pressure-sensitiveadhesive or in a variant, the pressure-sensitive adhesive may betransferred from a release liner with which the coated substrate iscombined.

Another object of the present inventions concerns a printed filmcomprising a substrate of which at least one face is coated with asurface layer comprising a water dispersible polymer and anethylenically unsaturated compound, said coated face of the substratebeing printed by conventional methods such as offset printing, gravure,flexography, screen process printing and letterpress printing usingradiation curable ink and subsequently radiation cured.

Ink formulations for radiation curing contains generally pigments,vehicle, solvent and additives. The solvents in these systems arelow-viscosity monomers, capable of reacting themselves (i.e., used asreactive diluents). The vehicle is usually composed of a resin derivedfrom unsaturated monomers, prepolymers or oligomers such as acrylatesderivatives which are able to react with the ethylenically unsaturatedcompound of the surface layer. For a UV ink, the “additives” contain alarge amount of photo initiators which respond to the photons of UVlight to start the system reacting.

A UV ink formulation may be generalized as:

pigment 15–20% prepolymers 20–35% vehicle 10–25% photoinitiators  2–10%other additives  1–5%.

For an electron beam curable ink, the “additives” contain generally nophotoinitiator.

The low viscosity monomers, sometimes termed diluents, are capable ofchemical reactions which result in their becoming fully incorporatedinto the ultimate polymer matrix.

The vehicle provides the “hard resin” portion of the formulation.Typically, these are derived from synthetic resins such as for example,urethanes, epoxides, polyesters which have been modified by reactionwith compounds bearing ethylenic groups such as for instance(meth)acrylic acid, hydroxyethyl(meth)acrylate reaction product ofcaprolactone with unsaturated compounds bearing a hydroxyl group, andthe like.

Appropriate adjustments could be made in the selection of theprepolymers and monomers used in order to achieve the requiredviscosities for the different methods of application.

Another aspect of the invention relates to a process for the manufactureof a printed film comprising the steps of

a) coating a substrate with an aqueous dispersion comprising a waterdispersible polymer and an ethylenically unsaturated compound;

b) drying the coating so obtained;

c) inking the dried coating with radiation-curable ink;

d) curing the ink with UV or EB radiations.

It is to be noted that the different steps of this process may beeffected in the same conditions, such as speed and costs, as withconventional surface layer.

Finally, the invention concerns also a printed film as obtained byinking a printable film according to the invention, and especially aprinted label so obtained.

The following Examples are given for the purpose of illustrating thepresent invention.

In these Examples, the determination of certain, characteristic valueswas carried out in accordance with the methods described below.

The adhesion of the radiation cured ink to the film in boiling waterconditions (simulating pasteurization) is controlled according to thefollowing procedures.

(A) A stirred waterbath is heated to 95° C. Once the temperature remainsstable, the sample of the printed film to be tested is immersed into thewater. After 45 minutes, the sample is removed from the waterbath andscratched with moderate pressure with a coin held square to the surfaceof the sample. The loss of print is reported as “pass” or “fail” wherein“pass” means no loss of print and “fail” means noticeable loss of print.

(B) The sample is removed from the waterbath at 95° C. after 45 minutesas described in A.

An adhesive tape is applied on the sample and then the adhesive tape asquickly removed. In this way, the properties of the ink which is printedon the printing sheet are observed.

The surface percentage of ink removed (visual estimation) is reported.

The adhesion of the radiation cured ink to the film in freezing waterconditions (simulating ice chest immersion) is controlled according tothe following procedure.

A waterbath containing a mixture of ice and water (50:50) is cooled to0° C. Once the temperature remains stable, the sample of the printedfilm to be tested is immersed into the water.

After 24 hours, the sample is removed from the waterbath and scratchedwith moderate pressure with a coin held square to the surface of thesample. The loss of print is reported as “pass” or “fail” in the samemanner as described above.

Example 1

A corona discharge treated polypropylene film of 50 mm thickness isprimed on one face at 0.2 g/m² with a polyurethane acrylate to which anisocyanate crosslinker is added prior to coatings. The primed face ofthe substrate is subsequently overcoated, at 1.0 g/m² and by reversegravure, with an aqueous dispersion containing 21.0 kg of an aliphaticpolyester based polyurethane (DAOTAN VTW 1238 from HOECHST; solidscontent 50%) which represents 80.8% (dry weight) of the aqueousdispersion, 0.9 kg of tripropylene glycol diacrylate (solids content100%; 7% (dry weight) of the aqueous dispersion), 1.1 kg of ammoniumzirconium carbonate (solids content 20%; 1.7% (dry weight) of theaqueous dispersion), 4.3 kg of colloidal silica (LUDOX HS40 from DUPONT; solid content 30%; 10% (dry weight) of the aqueous dispersion) and0.65 kg of silica gel as antiblocking agent (GASIL HP 250 fromCROSFIELD; solids content 10%; 0.5% (dry weight) of the aqueousdispersion).

The coated film is then dried in a hot air oven.

The coated film so obtained is then printed in a screen printing processwith RSP series ink (from NORCOTE), at 12 g/m².

The printed film so obtained is then UV cured with a medium pressuremercury vapour lamp (120 W/cm) at 122 m/min.

The printed film finally obtained is tested according to the methodgiven above. The results are reported in Table 1.

Comparative Example A

A corona discharge treated polypropylene film (but not primed) of 50 mmthickness, is printed and then cured as described in Example 1.

The printed film finally obtained is tested according to the methodgiven above. The results are reported in Table 1.

Comparative Example B

A printed film is prepared exactly as described, at Example 1, exceptthat the aqueous dispersion does not contain tripropylene glycoldiacrylate. The printed film finally obtained is tested according to themethod given above. The results are reported in Table 1.

TABLE 1 Freezing water Boiling water condition condition Example Scratchtest % ink removal Scratch test 1 Pass  0% Pass A* Fail 95% Fail B* Fail60% Fail *by way of comparison

These results show that the printable film according to the inventionproduces highly desirable properties with respect to adhesion of theradiation curable ink to the substrate. Especially, the influence of thepresence of the surface layer may be noted from the comparison betweenexamples A and B and the influence of the ethylenically unsaturatedcompound may be noted from the comparison between examples B and 1.

Example 2

A printed film is prepared exactly as described at example 1, exceptthat the ethylenically unsaturated compound is replaced with triacrylateof ethoxylated trimethylpropane (EBECRYL 1160 from UCB, S.A.).

Example 3

A printed film is prepared exactly as described at example 1, exceptthat the urethane polymer is replaced with an anionic acrylic polymer(NEOCRYL XK-90 from ZENECA).

Example 4 TO 6

Printed films are prepared exactly as described at example 2, exceptthat the urethane polymer is replaced respectively with an anionicacrylic polymer (NEOCRYL XK-90 from ZENECA) (example 4), with a styreneacrylic copolymer (GLASCOL LE 31 from ALLIED COLLOIDS) (example 5) andwith another anionic acrylic polymer (NEOTAC A-572 from ZENECA) (example6).

The printed films obtained at examples 2 to 6 are tested according tothe method given above and the results are reported in Table 2.

TABLE 2 Freezing water Boiling water condition condition Example Scratchtest % ink removal Scratch test 2 Pass 0 Pass 3 Pass 0 Pass 4 Pass 0Pass 5 Pass 0 Pass 6 Pass 0 Pass

These results show that a wide variety of surface layer andethylenically unsaturated compounds may be used.

1. A printable film comprising a polyolefin substrate and at least oneuncured surface layer printable by a radiation curable ink, said layerbeing a covering at least one face of said substrate, the majorcomponent of the layer being a water-dispersible acrylic polymer able toprovide a smooth film and open surface, the layer further containing (a)an ethylenically unsaturated compound having 2 to 5 ethylenical bondsper molecule in the form of a trifunctional acrylate in an amount of 2to 15% by weight of the water dispersible acrylic polymer and (b) across linking agent in the form of a coordinated metal ligand in anamount of 1 to 5% by weight of the water dispersible polymer, said layernot containing an addition polymerization photoinitiator, wherein thesubstrate is an oriented polypropylene film provided on both surfaceswith skin layers and a polyurethane acrylate primer is applied betweenthe surface layer and a skin layer.
 2. A film according to claim 1,wherein the trifunctional acrylate is a triacrylate of ethoxylatedtrimethyl propane.
 3. A film according to claim 1, wherein the waterdispersible acrylic polymer consists essentially of a homopolymer of(meth)acrylic acid or alkyl (meth)acrylate, said alkyl having 1 to 10carbon atoms, or a copolymer of two or more of said monomers.
 4. A filmaccording to claim 1, wherein the crosslinking agent is zirconiumammonium carbonate.